The present invention relates to high efficiency particulate filter materials, particularly HEPA type synthetic fiber filtration medium.
High efficiency particulate air (HEPA) filtration medium is commonly used in environments that require very clean conditions such as surgical operating rooms or clean rooms. More recently, HEPA filtration has become popular in uses having less demanding criteria such as heating, ventilation and air conditioning systems (HVAC systems), respiratory filters, vacuum cleaners, room air cleaners and the like. Conventional HEPA filtration medium comprises an extremely fine diameter fiberglass medium, generally formed from glass fibers having mean diameters of less than 1 micron. Filtration efficiency of glass fiber HEPA medium generally increases as the mean fiber diameter decreases at the expense of a corresponding increase in pressure drop across the filter. Pressure drops for glass fiber filtering medium with HEPA-type performance generally is in the range of 25mm H.sub.2 O or higher. Problems with this glass fiber filter medium include the aforementioned relatively high pressure drops, clogging due to surface loading of particles, and health concerns. In an attempt to address the pressure drop problems and other difficulties associated with fiberglass HEPA filtration medium, U.S. Pat. No. 4,824,451 proposes forming a near to HEPA performance type filter having efficiencies ranging from 95% to 99.26% at pressure drops of about 8mm H.sub.2 O, using meltblown microfiber webs having extremely small effective fiber diameters. Increased efficiencies were obtained by subsequently calendering the webs, such that efficiencies of up to 99.57% could be reached at pressure drops of from around 10mm to 20mm H.sub.2 O. Although the filter medium of this patent has lower pressure 5 drops than the fiberglass HEPA filter medium, the efficiency does not quite reach HEPA performance even at the relatively high basis weight of 2 oz/square yard (approximately 70 gm/m.sup.2) .
Extremely fine solution blown fibers are described in U.S. Pat. Nos. 4,011,506 and 4,650,506. As described in U.S. Pat. No. 4,650,506, these extremely fine solution blown fiber medium can be quite thin and have HEPA performance, however, lack sufficient strength, and as such are non-handleable and non-self supporting.
HEPA type filtration performance can also be obtained by simply increasing the basis weight or thickness of basically any filtration medium sufficiently. This approach is described in U.S. Pat. No. 5,240,479. This patent describes laminating four or more layers of blown microfiber filtration medium each having an effective fiber diameter of approximately 3.2 microns, a basis weight of 30 g/m.sup.2, and a thickness of 0.009 inches. Each particular layer is indicated as being 88% efficient (against 0.3 micron particles at 10.5 feet/minute). By placing four(4) of these filtration media in successive layers, filtration efficiencies of HEPA type performance are alleged obtained. The reported filter medium had a pressure drop of approximately 0.4 inches H.sub.2 O (10 mm H.sub.2 O), using over 120 g/m.sup.2 of electret charged filter medium. This filter is generally quite thick and difficult to pleat into a high density pleated structure.
Others have also purported that HEPA type filtration performance can be obtained by charged meltblown microfiber webs. For example, the article "Electret HEPA Filter", presented at INDA's American Filtration Society Joint Meeting, March 1993, Philadelphia, Pa. describes 150 millimeter thick filter webs having pressure drops of from 12 to 12.5mm of water at an airflow rate of 17 m.sup.3 /minute and having HEPA type performance. These electret charged blown microfiber (BMF) filters outperform traditional glass HEPA filters in providing for lower pressure drops and longer useful lives, particularly when used with a pre-filter. However, these media are still quite thick and further improvements in the performance of these types of webs would be desirable. A similar HEPA type performance BMF web is also described in "Nonwovens in Filtration: A Growing Market", a paper presented at INDA's Filtration Conference, Mar. 12-14, 1991, Philadelphia, Pa., which paper describes a medium exhibiting HEPA performance having a basis weight of approximately 100 g/m.sup.2, a thickness of 0.64mm, and a pressure drop of 8.5mm H.sub.2 O at a face velocity of 1.5 m/min. A similar web is also described in "Toyobo Electret Air Filter; Elitolon", a paper published by the Toyobo Company of Osaka, Japan on April, 1991. These filtration medium, although providing HEPA type performance, do so at relatively high basis weights of 100 gm/m.sup.2 or higher which filter webs are generally relatively thick and are more difficult to handle and form into complex pleated structures. There is significant desire to provide HEPA level performance filter medium at ever increasing efficiencies at lower pressure drops, lower basis weights and lower thickness ranges in a form which is easily pleatable and joinable to other functional layers.